Another crucial aspect for me is that Farage actually looks, sounds and acts like a politician. He doesn't automatically, at least from his appearance, come across as being 'extreme'. In short, he will have a certain degree of sway with and appeal to Middle England.
The extreme left have got to very careful, IMHO, as to how they combat the very real popularity being gained by UKIP. They're in danger of making Farage and his boys look like a viable option at the moment.